For detecting a disconnection of signal wires connecting a resolver and an electronic circuit, which detects a COS-phase signal and a SIN-phase signal outputted from the resolver, a relation that a sum of squares of these phase signals equals 1 (cos2 θ+sin2 θ=1) is used. That is, the disconnection is detected when the sum of squares does not equal 1. JP 2000-166205 discloses one exemplary technology of detecting inter-phase short-circuit in a resolver.
However, when a SIN-phase signal is “0” and a wire of the SIN-phase signal is disconnected under a state that a motor is not in rotation, for example, cos2 θ equals 1. For this reason, it is not possible to detect the disconnection unless the motor is in rotation.